Recently, due to the development of genetic analytical techniques, the use of genetic diagnoses for diagnosing the causes of diseases and estimating the appearance of diseases has been increased. In the genetic diagnoses, various methods for amplifying genes, represented by a PCR (Polymerase Chain Reaction) method, are used so as to detect a gene as a target.
For detecting a gene, for example, a multichannel photodetector disclosed in JP 2002-515602 A (see pages 23 to 40 and FIGS. 1 to 9) is used. FIG. 5 is a view schematically showing a structure of a conventional multichannel photodetector. The multichannel photodetector shown in FIG. 5 amplifies a gene by controlling a temperature of a sample that is mixed with fluorochrome, subsequently irradiates the sample with light beams, and receives fluorescence excited by the irradiation, thereby conducting an analysis.
As shown in FIG. 5, the multichannel photodetector mainly is composed of a reaction unit 41, a light source unit 42 and a photoreceptive unit 43. In FIG. 5, other components except these are omitted.
In an inside of the reaction unit 41, a sample mixed with reaction reagents, fluorochrome and the like is added. In addition, the reaction unit 41 is provided with a temperature control system (not shown in the figure) for performing the above-described gene amplification.
The light source unit 42 is provided with LEDs 44a to 44d that are different in wavelength of an emitted light beam. Thus, the light source unit 42 can change the wavelength of the emitted light beam according to the fluorochrome that is mixed with the sample. In addition, the light source unit 42 is provided with filter sets 47a to 47d that transmit only light beams with certain wavelengths, and condensing lenses 46a to 46d. 
Moreover, the light source unit 42 also includes dichroic mirrors 48a to 48e so that the light beams emitted by the respective LEDs (44a to 44d) may pass through a lens 49 and an output window 50 provided in a housing 45, and may enter the reaction unit 41. Furthermore, the LEDs 44a to 44d, the filter sets 47a to 47d, the condensing lenses 46a to 46d and the dichroic mirrors 48a to 48e are arranged in the housing 45 so that energy of the light beams emitted by the respective LEDs (44a to 44d) may be constant.
The photoreceptive unit 43 includes four photoreceptors 51a to 51d, because wavelengths of the excited fluorescence vary according to the kinds of fluorochrome. Moreover, the photoreceptive unit 43 is provided with filter sets 52a to 52d that transmit only light beams with certain wavelengths and lenses 53a to 53d, where filter sets 52a to 52d and lenses 53a to 53d respectively are included in the photoreceptors (51a to 51d).
Furthermore, the photoreceptive unit 43 is provided with dichroic mirrors 54a to 54e. Thus, the light beams, which are output from the reaction unit 41 and pass through an entrance window 57 provided in a housing 55 and a lens 56, pass through or are reflected by some of the dichroic mirrors and enter the corresponding photoreceptors (51a to 51d), according to the respective wavelengths of the light beams.
As mentioned above, the multichannel photodetector shown in FIG. 5 can output and receive light beams with different wavelengths, and thus can select wavelengths corresponding to the kinds of the used fluorochrome so as to detect a gene.
By the way, in genetic diagnoses, the number of diagnostic items may further increase due to the development of the genetic analytical techniques in the future, and accordingly, additional kinds of fluorochrome for being mixed with a sample may be introduced. Moreover, according to the exploitation of new kinds of fluorochrome in the future, the number of kinds of fluorochrome that can be applied to genetic diagnoses may increase. In such cases, it is required for multichannel photodetectors to have a capability of irradiating with light beams with wavelengths that correspond to the newly applied kinds of fluorochrome.
Moreover, the multichannel photodetector shown in FIG. 5 also may be utilized for fluorometry using kinds of fluorochrome other than the kinds used for detecting genes. In this case, the multichannel photodetector is required to be capable of irradiating with light beams with wavelengths that correspond to the newly applied kinds of fluorochrome.
However, the above-mentioned multichannel photodetector shown in FIG. 5 has a disadvantage that the LEDs 44a to 44d, the filter sets 47a to 47d, the condensing lenses 46a to 46d and the dichroic mirrors 48a to 48e have a complicated arrangement, in spite of having an advantage of allowing the energy of the light beams emitted by the light source unit 42 to be constant.
Therefore, there is a problem that it is structurally difficult to add LEDs to the multichannel photodetector, and accordingly, the multichannel photodetector cannot be adapted to a case where such new kinds of fluorochrome are introduced for use. This problem is also applicable to the photoreceptive unit 43.
Moreover, in the above-mentioned multichannel photodetector shown in FIG. 5, the number of dichroic mirrors necessary for outputting light beams from the light source unit 42 is larger than the number of variations of the wavelengths. Similarly, the number of dichroic mirrors necessary for leading the light beams that enter the photoreceptive unit 43 to the respective photoreceptors is also larger than the number of the variations of the wavelengths. Accordingly, the above-mentioned multichannel photodetector shown in FIG. 5 has a problem of difficulty in reducing cost.
The object of the present invention is to solve the above-mentioned problems, and to provide a light source unit in which light emitting devices can be added or removed easily, a photoreceptive unit in which photoreceptors can be added or removed easily, and a multichannel photodetector using the light source unit and the photoreceptive unit.